PATE is a crystalline thermoplastic resin excellent in heat resistance, chemical resistance, flame retardance, stiffness, etc. These properties are improved by increasing the degree of crystallinity of PATE.
Formed products of PATE generally exhibit a tendency to lower their toughness, impact resistance, etc. as their degrees of crystallinity are increased. Besides, PATE improved in crystallinity is inferior in melt extrudability because its melt viscosity rapidly decreases upon its melt extrusion and the rate of crystallization in its cooling process after the extrusion becomes faster. More specifically, such a PATE is accompanied by problems of great drawdown and poor shape retention upon its melt extrusion, and also breaking of an extrudate upon taking off through a die, lowered dimensional accuracy of extruded products, etc.
Therefore, various forming processes have been investigated and attempted in order to overcome such drawbacks as to the extrudability of PATE and the physical properties of its extruded products.
For example, it has been proposed in Japanese Patent Application Laid-Open No. 47590/1984 to use, as a core material, a pipe made of a poly(phenylene sulfide) (hereinafter abbreviated as "PPS") and cover the core material with a fiber-reinforced thermosetting resin into a composite resin pipe, thereby offsetting the insufficient strength of the PPS pipe.
In Japanese Patent Application Laid-Open No. 85747/1984, it has been proposed to insert a PPS pipe inside a metallic pipe and join them to each other into a composite pipe, thereby improving the insufficient strength and stiffness of the PPS pipe.
It has been disclosed in Japanese Patent Application Laid-Open No. 145131/1984 to offset the insufficient strength of a PPS pipe by applying a thermoplastic resin other than PPS, for example, rigid polyvinyl chloride, polypropylene, nylon or the like, outside the PPS pipe.
In order to improve poor formability of PPS, such as a difficulty of shape retention due to the lowered melt viscosity upon its extrusion, or occurrence of cracks, which is contingent to the crystallization and shrinkage upon its compression molding, it has been disclosed in Japanese Patent Application Laid-Open No. 198120/1984 to unite reinforcing fibers of at least 3 mm in length to the PPS and then wind the resulting composite material (sheet-like material) around a mandrel while heating it to a temperature at which PPS is melted to compression-mold the composite, thereby producing a tubular article.
However, these known processes are all inferior in productivity due to their complexity in forming operation, and can improve neither the melt extrudability of PPS itself nor the physical properties of resulting formed products.
On the other hand, it has been disclosed in Japanese Patent Application Laid-Open No. 90216/1987 to use a PATE having a melt viscosity of 2,000-40,000 poises (at 310.degree. C. and a shear rate of 200 sec.sup.-1) and a substantially linear structure for preventing drawdown upon melt forming, thereby producing a tubular extruded article. This PATE is somewhat inferior in crystallinity and hence still insufficient in dimensional stability with crystallization upon forming.